This invention relates generally to synthetic array mapping processors, and more particularly to a radar telescope for providing high resolution imagery of relatively large areas at short ranges.
In synthetic array mapping, radar data received from selected range resolution elements within an illuminating beam is periodically sampled as the antenna is moved along a flight path. This data is then electronically focused to simulate the physical focus (narrow azimuth beam width) of an antenna having a length approximately equal to the flight path segment over which the synthetic array was formed. Basically, the electronic focusing involves amplitude weighting and phase adjustment of a sequence of range gated radar returns to cause the returns from a particular path of ground to be accentuated while those from other contiguous ground patches are attenuated. Due to aircraft motion during the time required to collect the data, a phase history is impressed on the returns from each target; and for these returns to add in phase requires that phase compensation be applied thereto.
One processing technique, sometimes referred to as "batch" processing, applies a phase adjustment which "tracks out" the doppler frequency of the center point of the map area. Filter banks, responsive to the doppler frequency differential across the target area, are then utilized to provide signals indicative of the radar reflectivity characteristics of the mapped area.
Another processing technique, sometimes referred to as "line-by-line" processing, accomplishes the required phase correction by applying the proper phase adjustments to a sequence of range gated radar returns so that they focus on a particular ground point. The sequence is then advanced one range resolution element across the aircraft track (range dimension) and the proper phase adjustments reapplied. This process is repeated until all range elements across the swath have been operated upon, at which time the sequence starts back at the first range element and advances one azimuth element along the aircraft track (azimuth dimension). In some applications parallel processing channels are used so that the different azimuth elements are processed simultaneously for each range interval.
A significant advantage of "batch" processing is that it is particularly well adapted to digital implementations; and with innovations such as the Fast Fourier Transforms techniques (sometimes referred to as the "Cooley-Tukey algorithm") the number of mathematical operations required to generate a block of N azimuth resolution elements is reduced from N.sup.2 for line-by-line processing to 2N log .sub.2 N for the batch processing technique. Batch processors have, however, several disadvantages--particularly for reconnaissance applications. One of these is related to the geometry of batch processing inasmuch as for reasonably simple mechanizations, the angular and not the azimuth resolution is constant across a block of data. For this reason the block of data tends to have a keystone shape which makes it very difficult to generate a composite map by fitting separate blocks together; and the additional display processing caused thereby increases the complexity of "batch" type processor systems.